# std::replace, std::replace_if

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class ForwardIt, class T > void replace( ForwardIt first, ForwardIt last, const T& old_value, const T& new_value ); |  | (constexpr since C++20) (until C++26)
template< class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > constexpr void replace( ForwardIt first, ForwardIt last, const T& old_value, const T& new_value ); |  | (since C++26)
| (2) |
template< class ExecutionPolicy, class ForwardIt, class T > void replace( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& old_value, const T& new_value ); |  | (since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > void replace( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& old_value, const T& new_value ); |  | (since C++26)
| (3) |
template< class ForwardIt, class UnaryPred, class T > void replace_if( ForwardIt first, ForwardIt last, UnaryPred p, const T& new_value ); |  | (constexpr since C++20) (until C++26)
template< class ForwardIt, class UnaryPred, class T = typename std::iterator_traits <ForwardIt>::value_type> > constexpr void replace_if( ForwardIt first, ForwardIt last, UnaryPred p, const T& new_value ); |  | (since C++26)
| (4) |
template< class ExecutionPolicy, class ForwardIt, class UnaryPred, class T > void replace_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred p, const T& new_value ); |  | (since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred, class T = typename std::iterator_traits <ForwardIt>::value_type> > void replace_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred p, const T& new_value ); |  | (since C++26)

Replaces all elements in the range `[`first`,` last`)` with new_value if they satisfy specific criteria.

1) Replaces all elements that are equal to old_value (using operator==).
3) Replaces all elements for which predicate p returns true.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If *first = new_value is invalid(until C++20)new_value is not writable to first(since C++20), the program is ill-formed.

### Parameters

first, last | - | the range of elements to process
old_value | - | the value of elements to replace
policy | - | the execution policy to use
p | - | unary predicate which returns ​true if the element value should be replaced. The expression p(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `ForwardIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy(since C++11). ​
new_value | - | the value to use as replacement
Type requirements
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `UnaryPred` must meet the requirements of Predicate.

### Return value

(none)

### Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N comparisons using operator==.
3,4) Exactly \(\scriptsize N\)N applications of the predicate p.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Notes

Because the algorithm takes old_value and new_value by reference, it can have unexpected behavior if either is a reference to an element of the range `[`first`,` last`)`.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1-4)

### Possible implementation

replace
    template<class ForwardIt,
             class T = typename std::iterator_traits<ForwardIt>::value_type>
    void replace(ForwardIt first, ForwardIt last,
                 const T& old_value, const T& new_value)
    {
        for (; first != last; ++first)
            if (*first == old_value)
                *first = new_value;
    }
replace_if
    template<class ForwardIt, class UnaryPred,
             class T = typename std::iterator_traits<ForwardIt>::value_type>
    void replace_if(ForwardIt first, ForwardIt last,
                    UnaryPred p, const T& new_value)
    {
        for (; first != last; ++first)
            if (p(*first))
                *first = new_value;
    }

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <complex>
    #include <functional>
    #include <iostream>

    void println(const auto& seq)
    {
        for (const auto& e : seq)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    int main()
    {
        std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};

        // Replace all occurrences of 8 with 88.
        std::replace(s.begin(), s.end(), 8, 88);
        println(s);

        // Replace all values less than 5 with 55.
        std::replace_if(s.begin(), s.end(),
                        std::bind(std::less<int>(), std::placeholders::_1, 5), 55);
        println(s);

        std::array<std::complex<double>, 2> nums{{{1, 3}, {1, 3}}};
        #ifdef __cpp_lib_algorithm_default_value_type
            std::replace(nums.begin(), nums.end(), {1, 3}, {4, 2});
        #else
            std::replace(nums.begin(), nums.end(), std::complex<double>{1, 3},
                                                   std::complex<double>{4, 2});
        #endif
        println(nums);
    }

Output:

    5 7 4 2 88 6 1 9 0 3
    5 7 55 55 88 6 55 9 55 55
    (4,2), (4,2)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 283 | C++98 | `T` was required to be CopyAssignable (and EqualityComparable for `replace`), but the value type of `ForwardIt` is not always `T` and `T` is not always writable to `ForwardIt` | required *first = new_value to be valid instead

### See also

replace_copyreplace_copy_if | copies a range, replacing elements satisfying specific criteria with another value (function template)
ranges::replaceranges::replace_if (C++20)(C++20) | replaces all values satisfying specific criteria with another value (algorithm function object)

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